1. The Field of the Invention
This invention relates to geosynthetic reinforcements for reinforced earthwork construction and, more particularly, to novel systems and methods for using fine-grained fill in structural applications.
2. The Background Art
In general, earthen fills may be divided into two groups, namely granular and fine-grained. Granular fill such as gravel, coarse sand, fine sand, and the like is sometimes referred to as cohesionless fill. Granular fill has an inherent ability to resist shear loads. Fine-grained fill, on the other hand, is often referred to as cohesive fill because it is held together primarily by cohesion between particles. Fine-grain fill has low particle-to-particle frictional interaction and thus, little inherent ability to resist shear loads. Examples of fine-grained fill include silt and clay.
Fine-grained fill is typically not included in the design and construction of geosynthetic reinforcements (e.g. retaining walls, berms, breakers, road beds, causeways, etc.) because of its low shear strength and the potential for settlement, slope failure, bulging failure, and creep. Fine-grained fill is particularly avoided in geosynthetic reinforcements that will be exposed to excess water.
For example, the amount of fine-grained fill currently tolerated behind modern reinforced retaining walls is strictly limited. In recent years, conventional concrete, gravity, and cantilever retaining wall designs have been rendered obsolete by geosynthetics and other materials used to reinforce the fill located behind retaining walls. Retaining walls reinforced with geosynthetics are significantly cheaper to construct, support greater heights on poorer quality foundations, and appear to have greater seismic stability.
Current retaining walls reinforced with geosynthetics require fill containing significant amounts of granular material. This granular component increases the shear strength of the fill to the point where it can engage and retain the geosynthetic reinforcements. In its specified retaining wall designs, the National Concrete Masonry Association (NCMA) limits the percentage of fines (earthen particles sufficiently small to pass through a number two hundred sieve) within backfill to fifteen percent by weight. The American Association of State Highway and Transportation Officials (AASHTO) limits the percentage of fines within similar backfill to twenty-five percent by weight.
In general, granular fill is more expensive than fine-grained fill. Additionally, in certain locations, the availability of fine-grained fill far exceeds that of granular fill. When granular fill is needed in such locations, it must be transported. The cost associated with such transportation may be high, and occasionally prohibitive. Accordingly, what is needed is an apparatus and method supporting the use of fine-grained or high-moisture-containing fill in geosynthetic reinforcements such as retaining walls, berms, breakers, roadbeds, causeways, and the like.